probo/models/quiz.go

package models

import (
	"crypto/sha256"
	"errors"
	"fmt"
	"io"
	"sort"
	"strings"

	"gopkg.in/yaml.v2"
)

type Quiz struct {
	Meta

	Hash       string    `json:"hash"`
	Question   *Question `json:"question" gorm:"foreignKey:ID"`
	Answers    []*Answer `json:"answers" gorm:"many2many:quiz_answers"`
	Tags       []*Tag    `json:"tags" yaml:"-" gorm:"-"`
	Correct    *Answer   `json:"correct" gorm:"foreignKey:ID"`
	CorrectPos uint      `gorm:"-"` // Position of the correct answer during quiz creation
	Type       int       `json:"type"`
}

func MarkdownToQuiz(markdown string) (*Quiz, *Meta, error) {
	meta, remainingMarkdown, err := ParseMetaHeaderFromMarkdown(markdown)
	if err != nil {
		return nil, nil, err
	}

	lines := strings.Split(remainingMarkdown, "\n")

	questionText := ""
	answers := []*Answer{}

	for _, line := range lines {
		if strings.HasPrefix(line, "*") {
			answerText := strings.TrimPrefix(line, "* ")
			answer := &Answer{Text: answerText}
			answers = append(answers, answer)
		} else {
			if questionText != "" {
				questionText += "\n"
			}
			questionText += line
		}
	}

	questionText = strings.TrimRight(questionText, "\n")

	if questionText == "" {
		return nil, nil, fmt.Errorf("Question text should not be empty.")
	}

	if len(answers) < 2 {
		return nil, nil, fmt.Errorf("Number of answers should be at least 2 but parsed answers are %d.", len(answers))
	}

	question := &Question{Text: questionText}
	quiz := &Quiz{Question: question, Answers: answers, CorrectPos: 0}

	if meta != nil {
		quiz.Meta = *meta
	}

	return quiz, meta, nil
}

func QuizToMarkdown(quiz *Quiz) (string, error) {
	if quiz.Question == nil {
		return "", errors.New("Quiz should contain a question but it wasn't provided.")
	}

	if len(quiz.Answers) < 2 {
		return "", errors.New("Quiz should contain at least 2 answers but none was provided.")
	}

	quiz.Correct = quiz.Answers[quiz.CorrectPos]

	if quiz.Correct == nil {
		return "", errors.New("Quiz should contain a correct answer but not was provided.")
	}

	correctAnswer := "* " + quiz.Correct.Text
	var otherAnswers string

	for pos, answer := range quiz.Answers {
		if quiz.CorrectPos != uint(pos) {
			otherAnswers += "* " + answer.Text + "\n"
		}
	}

	markdown := quiz.Question.Text + "\n\n" + correctAnswer + "\n" + otherAnswers

	return markdown, nil
}

func (q *Quiz) GetID() string {
	return q.ID
}

func (q *Quiz) SetID(id string) {
	q.ID = id
}

func (q *Quiz) GetHash() string {
	return q.calculateHash()
}

func (q *Quiz) calculateHash() string {
	result := make([]string, 0)

	result = append(result, q.Question.GetHash())

	for _, a := range q.Answers {
		result = append(result, a.GetHash())
	}

	orderedHashes := make([]string, len(result))

	copy(orderedHashes, result)
	sort.Strings(orderedHashes)

	return fmt.Sprintf("%x", sha256.Sum256([]byte(strings.Join(orderedHashes, ""))))
}

func ParseMetaHeaderFromMarkdown(markdown string) (*Meta, string, error) {
	reader := strings.NewReader(markdown)
	var sb strings.Builder
	var line string
	var err error
	for {
		line, err = readLine(reader)
		if err != nil {
			if err == io.EOF {
				break
			}
			return nil, "", err
		}

		if strings.TrimSpace(line) == "---" {
			break
		}
	}

	for {
		line, err = readLine(reader)
		if err != nil {
			if err == io.EOF {
				break
			}
			return nil, "", err
		}

		if strings.TrimSpace(line) == "---" {
			break
		}

		sb.WriteString(line)
	}

	if sb.String() == "" {
		return nil, markdown, nil
	}

	var meta Meta
	err = yaml.Unmarshal([]byte(sb.String()), &meta)
	if err != nil {
		return nil, markdown, err
	}

	remainingMarkdown := markdown[strings.Index(markdown, "---\n"+sb.String()+"---\n")+len("---\n"+sb.String()+"---\n"):]

	return &meta, remainingMarkdown, nil
}

func readLine(reader *strings.Reader) (string, error) {
	var sb strings.Builder
	for {
		r, _, err := reader.ReadRune()
		if err != nil {
			if err == io.EOF {
				return sb.String(), io.EOF
			}
			return "", err
		}

		sb.WriteRune(r)
		if r == '\n' {
			break
		}
	}

	return sb.String(), nil
}